It is not necessary to switch versions every time, it's just 'recommended' to use these versions for the specific server. It depends on what server you are on and what version you are in.
For example,
if you were just on Olympus (1.12.2) and want to go to parkour (1.8.9), there might be a few slight glitches such as lily pads, but you will be fine.
I suggest testing out server and version combinations to discover what versions the servers you play support. However, I still recommend using the preferred versions on the different servers as glitches will not happen as often.
best wishes <3
-kxe